1. Field of the Invention
The present invention relates to an aqueous polyurethane dispersion, and more particularly to a process for preparing solvent-free aqueous polyurethane dispersion.
2. Description of Related Art
In recent years, environmental protection has become increasingly prominent, and, regarding how to effectively reduce emission of volatile organic compounds (VOCs) and other pollutants have inevitably become a global trend. Particularly, applications related to aqueous polyurethane and aqueous acrylic are expected to reduce use of solvents, when the solvents if used could be one of prohibited pollutants.
Presently, aqueous polyurethane is synthesized through the known acetone process due to using acetone as a solvent in the course of manufacturing process, in which polyether or polyester polyol reacts with isocyanate so as to synthesize a high-viscosity prepolymer terminated by an NCO functional group, then adding a low-boiling solvent that is compatible to water (such as acetone, butanone or tetrahydrofuran) for dilution and viscosity reduction, afterward adding hydrophilic chain extender for chain extension, and adding water under high shear force for emulsification. As water is added, phase transition increases, so as to form an aqueous polyurethane system in which water becomes a continuous phase. At last, the solvent is distilled, separated and recycled.
One available hydrophilic chain extender for aqueous polyurethane produced from the known acetone process is selected from carboxylates, such as dimethylol propionic acid (DMPA) or dimethylolbutanoic acid (DMBA), and has to work with triethylamine (TEA) as a neutralizer. However, TEA being a toxic compound and having penetrating odor is against the product requirement for low odor.
In view of this issue regarding the acetone process due to using acetone can cause environmental pollution if the acetone is not recycled properly, an alternative approach to mix the prepolymer (hereinafter abbreviated as a prepolymer preparing approach) is addressed, which uses polyurethane (PU) prepolymer having a truncated NCO end for emulsification in the aqueous phase to eliminate the use of huge solvents.
However, in such a known prepolymer preparing approach, the viscosity of the prepolymer takes an important role. If the viscosity is high, dispersion of the prepolymer is difficult and even impossible. Thus, a small amount of solvents is usually used to reduce the viscosity. In addition, since water can react with isocyanate, so that the process of dispersion shall not last long, a chain extender has to be added at the earliest possibility, and the consumption of isocyanate will be too high; otherwise, an alternative is to use isocyanate that is less reactive. Therefore, the aqueous polyurethane (PU) made using this known prepolymer preparing approach is usually qualitatively poor than that made using the aforesaid acetone process.
Consequently, the known acetone process for preparing aqueous polyurethane provides good reproducibility of synthesis, but in course of manufacturing process the prepolymer is highly viscous and poorly dispersed, and the solvents can only be added when the reactant has been cooled, otherwise there may be a bumping.
Accordingly, the acetone process has disadvantages including use of huge acetone solvents, pollution to the environment when acetone solvents not distilled and recycled properly, needs for making an investment in acetone distilling and recycling facilities, and excessive residual of distilled solvents that prevents the solvents from being real water-borne as desired.
The prepolymer preparing approach can only use less reactive isocyanate, since the viscosity of aqueous polyurethane if prepared by the approach is limited to the dispersion of the prepolymer, so that the final PU product prepared by the approach is qualitatively inferior to the final PU product prepared by the aforesaid acetone process.